FIG. 1 generally illustrates the configuration of a static capacitance-to-voltage converter described in Laid-open Japanese Patent Application No. 61-1457. This static capacitance-to-voltage converter has been proposed to solve a problem of the prior art which suffers from the inability of accurate voltage conversion due to the fact that a stray capacitance of a cable used to connect an unknown static capacitance is superimposed on the unknown static capacitance, and that these static capacitances may vary due to movements and bending of the cable or the like. As illustrated in FIG. 1, an unknown capacitance Cx is connected between an alternating current (AC) signal generator OS and an operational amplifier OP with connection cables covered with shielding lines s to reduce the influence of stray capacitances Cs1, Cs2, Cs3. Specifically, an output and an inverting input of the operational amplifier OP are connected through a feedback circuit formed of a parallel circuit including a resistor Rf and a capacitor Cf. The unknown capacitance Cs has one end connected to the inverting terminal of the operational amplifier OP through a shielding line s, and the other end connected to the AC signal generator OS through another shielding line s. Both of the shielding lines and a non-inverting input of the operational amplifier OP are grounded.
With the configuration described above, since substantially no voltage difference exists between the two ends of the unknown capacitance Cx, the stray capacitance Cs2 is not charged. Also, since the stray capacitance Cs3 is regarded as a coupling capacitance of both the shielding lines s, the stray capacitance Cs3 can be eliminated by grounding the shielding lines s. In this way, the influence exerted by the stray capacitances of the cables for connecting the unknown capacitance Cx is reduced by using the shielding lines s, so that a charge equal to that induced on the unknown static capacitance Cx is induced on the capacitor Cf of the feedback circuit, resulting in an output proportional to the unknown static capacitance Cx produced from the operational amplifier OP. Stated another way, assuming that an output voltage of the AC signal generator OS is Vi, an output voltage Vo of the operational amplifier OP is expressed by -(Cx/Cf)Vi, so that the converter of FIG. 1 may be used to convert the unknown static capacitance Cx into the voltage Vo from which the unknown static capacitance Cx can be derived together with the known values Cf and Vi.